This invention relates to a silicon semiconductor strain gage, and in particular, to a silicon semiconductor strain gage comprising a thin layer of highly conductive silicon on a semi-insulating host substrate layer.
Silicon is commonly used as a semiconductor sensor material because it exhibits many large physical effects to various applied sensitivities or signals and permits cost-efficient batch fabrication of sensors. One very useful effect that silicon exhibits is piezoresistance. This effect is exhibited by silicon in response to an applied a mechanical strain. This effect is considerable in silicon because the average mobility of electrons and holes therein is largely affected by strain.
Numerous strain gages based on silicon semiconductor materials have been designed and are presently available. Many of these strain gages are fabricated from bulk single crystal silicon using conventional photolithography and etching techniques.
A major disadvantage of such semiconductor strain gages is that their strain gage resistance depends directly on their thickness. When manufactured in bulk, the strain gage resistance from gage to gage can vary by a factor of 3 or more because current etching processes are incapable of maintaining thickness uniformity, on the order of 0.0005 inches or 12 microns. This is mainly due to the pre-etching thickness of the gages which is typically about 0.006 inches and requires over 90% of the silicon to be removed. Overshooting the gage thickness by a mere 0.0001 inch results in a 20% change in the gage=s strain gage resistance.
Accordingly, a need exists for a silicon strain gage with improved uniformity of strain gage resistance.
In accordance with the invention, a semiconductor strain gage comprising an electrically resistive substrate layer and a layer of electrically conductive silicon supported by the substrate layer. The silicon layer can be an epitaxial silicon layer grown on a surface of the substrate layer or a diffused or ion-implanted layer formed in the surface of the substrate layer.
In further accordance with the invention, a force measuring and detecting device comprising a force responsive member and the above-described semiconductor strain gage attached to the force responsive member, the strain gage measuring forces applied to the force responsive member.